Field of the Invention
The present invention relates to a chair comprising:                a support structure;        a seat fixed to the support structure; and        a backrest connected to the seat by means of at least one upright of plastic material, wherein said at least one upright is elastically deformable to allow a movement of the backrest between a rest position and a backward tilted position.        
Description of Prior Art
The document EP-A-2497390 by the same Applicant describes a chair comprising a tilting backrest connected to the support structure by means of two uprights spaced apart in a transverse direction. Each of said uprights is provided with a plurality of generally transverse through-cuts relative to the longitudinal axis of the respective upright and spaced apart along this longitudinal axis. The cuts locally weaken the uprights and produce bending zones, which allow a bending of the uprights and a movement of the backrest between a rest position and a backward tilted position. The cuts divide the upright into a series of essentially non-deformable teeth that come into contact with each other in the backwards inclined position, so as to stop the backward tilt of the backrest.
When only the elasticity of a deformable material is used to obtain the bending of the uprights, it is important to provide for a limitation of the bending that prevents yielding of the material at the most stressed bending areas.
The major difficulties in the design of chairs with flexible uprights consist in the design of shapes of the flexible uprights that are harmonized with the line of the chair, capable of providing the backward bending of the backrest, to stop the bending in the position in which the required comfort is obtained, and to avoid excessive stresses of the material in the bending zones.